The present invention generally relates to noise reduction systems and, more particularly, to acoustic arrays applied on the underwater surfaces of ships.
Modern sonar arrays used on the exterior surfaces of submarine hulls are commonly formed of three distinct layers connected to each other in sandwich fashion. The three layers comprise a sound isolation baffle or skirt baffle attached to the exterior surface of the vessel; an intermediate sound conditioning layer secured over the isolation baffle; and an outer layer containing a plurality of hydrophone units.
The sound isolation baffles are used to eliminate or reduce shipboard noise which would otherwise adversely affect the performance of the hydrophone units. The sound isolation baffles in use today attenuate, absorb and reflect shipboard noise by means which may include:
a. acoustic reflecting layers containing a plurality of hollow, compliant tubes; PA1 b. gas and/or air filled multicellular materials such as elastomeric foams, wherein the size, number, and orientation of the microcells are critical factors affecting optimum performance of the baffle; PA1 c. composite materials such as "Corprene", wherein cork particles are embedded in a neoprene matrix; and PA1 d. elastomeric materials containing flattened steel tubes which are designed to resonate in their thickness mode at a preselected frequency.
However, the baffle systems outlined above suffer from various disadvantages. For example, the compliant tubes lose their efficiency when hydrostatic pressures cause the tube walls to collapse, and increasing the rigidity of the tubes to prevent such collapse of the tubes normally results in diminished acoustic performance. Further, for example, gas-filled multicellular materials normally exhibit random wall thickness and very high shape factors (ratio of the area on one face of the loaded compression element to the free lateral area). This relationship, which also applies to materials incorporating cork particles and air interstices, results in diminished performance at lower frequencies and higher pressures. Furthermore, baffles which utilize flattened steel tubes are acoustically effective in their design frequency range, but are prone to fatigue cracking and they become unduly heavy when designed for low frequency applications.
Baffle systems which primarily function as sound-absoring means also have operational limitations. For example, the acoustic impedance of sound absorbing baffles must match the acoustic impedance of the water, otherwise the sound energy will not enter the sound absorbing baffles and dissipate therein. Additionally, elastomeric materials in sound absorbing baffles must possess sufficient compliance so that the elastomeric materials will exhibit maximum motion at the desired frequencies to convert the incident sound energy into heat through hysteresis of the elastomer. When the hydrostatic pressures on the acoustic isolation baffles are increased, for example, the resultant increased stress in the elastomer generally results in a shift of its optimum sound absorption characteristics to higher sound frequencies and also results in an impedance mismatch with the water so that the amount of sound entering the baffle is reduced. Further, for low frequency applications this type of baffle is prohibitively thick.
The intermediate element of an acoustic array, the signal conditioning plate, ideally provides a high insertion loss to the incoming signals, with a minimum phase shift, and the conditioning plate relies heavily on the skirt baffle for pressure release. One form of conditioning plate comprises a plurality of aluminum plates supported in spaced relationship by steel springs and sealed together along the plate edges by a viscoelastic binder. Another type of conditioning plate consists of elastomeric elements encased within compartments in a rigid module to provide one-quarter wavelength resonance response to incident acoustic signals.
The outer layer of the acoustic array includes hydrophone elements embedded in a viscoelastic medium. Preferably, the viscoelastic medium has an impedance equivalent to the impedance of seawater in order to optimize acoustic receptivity.